The slow dance of galactic mergers

Call it a cosmic waltz. The Hubble Space Telescope has caught two galaxies circling each other, their spiral arms entwined. It's a relationship doomed to end in a spectacular merger with an outburst of star formation and the growth of a massive central black hole.

Many astronomers, including Bruce Elmegreen, think that's how galaxies grow. Equal partners merge their substance to form a greater entity. If the partners are unequal, the larger galaxy tears apart the smaller and gobbles up the debris.

For Dr. Elmegreen, who works at IBM's T.J. Watson research Center in Yorktown Heights, N.Y., this is a rare opportunity to catch the drama at the beginning of the first act.

He explains that, until now, astronomers have been like analysts reconstructing a car wreck. They can't tell what the cars looked like from the debris. In the image NASA released today, he says, astronomers can see what the galaxies look like as they begin their interaction.

"This is a test for many ideas" about what happened in the early universe, adds.

Bruce Elmegreen and Debra Elmegreen, who is with Vassar College in Poughkeepsie, N.Y., are leading an international research team to make the most of this opportunity.

The Hubble images, plus data from the Very Large Array Radio Telescope near Socorro, N.M., show galaxy IC 2163 (left) rotating counterclockwise around galaxy NGC 2207, 114 million light years from Earth. They came closest to each other 40 million years ago.

Bruce Elmegreen explains that "the forces acting on these galaxies are enormous." The images show the larger galaxy NGC2207 flopping about.

These are tidal forces like that with which the earth and moon pull on one another. Those bodies are too strong to be disrupted. Galaxies are more fragile. The tidal force of a large galaxy can tear up a smaller companion.

Bruce Elmegreen expects that, billions of years from now, the waltzing galaxies will crash together. Merging gases will spark a burst of star formation. Material spiraling down to the center will feed a black hole - an object so dense that nothing escapes its gravity.

In one computer-based simulation published Oct. 1 in Astrophysical Journal Letters, James Bullock with the University of California at Santa Cruz and colleagues show that galaxy growth by merger probably was common in the early universe.

"We believe that what we're seeing ... is the build-up of structure in the universe, and that this process generates stars with high efficiency," Dr. Bullock says.

Evidence published today in Nature magazine supports astronomers' belief that our own Milky Way galaxy has been growing in this manner.

Amina Helmi with the Leiden Observatory in the Netherlands and colleagues have identified streams of debris in our galaxy as remains of a small galaxy torn apart and devoured long ago.

Also, Young-Wook Lee with Korea's Yonsei University in Seoul and colleagues identify certain star clusters as remains of another small galaxy sucked into our own.

These ancient victims were small nearby galaxies. In the long run, our Milky Way will have to deal an equal rival. Bruce Elmegreen notes that the large Andromeda galaxy 2 million light years away, which had been moving off as part of the expansion of the universe, has "already turned around." It is heading right toward us as mutual gravitational attraction takes control.

However, he says that when the crash comes billions of years from now the stars will be so far apart our descendants will hardly notice it. They would "stay in the outer parts and enjoy the show," he says.

There will be lots more stars in the sky and a brighter Milky Way.

(c) Copyright 1999. The Christian Science Publishing Society

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